As noted earlier, most of the effort on high efficiency particulate traps have used monolithic ceramic traps having porous walls through which the exhaust gas passes and is filtered. While these traps remove 95–98% of the particulate, pressure drop across the traps builds up due to the accumulation of soot and ash. While the soot can be burned away by heating all, or a portion, of the exhaust gas, loss of energy occurs and, more seriously, the heat of the combustion of the soot leads to cracking and melting of the traps. In addition, the incombustible ash must be periodically removed by a disruptive and expensive cleaning operation.
In recent years, catalyst coatings have been applied to the wall flow traps to reduce the temperature at which the soot is ignited and to cause the soot to burn more often to reduce the amount of accumulated soot. This approach tends to prevent overheating of the traps during regeneration and reduces engine fuel consumption by igniting the soot all, or most of the time, by the heat of the engine exhaust. This system is called continuous regeneration technology (CRT) and has been used in a number of retrofit applications such as city and school buses. However, these applications must operate at a reasonably high average load factor and the engines must be in a condition that will meet the engine manufacturer's specification. To obtain the most reliable operation, the engines must use very low sulfur fuel to further reduce the light-off temperature of the soot. Finally, these systems are also subject to plugging by incombustible ash.
A more promising approach is to remove the soot and ash by physically removing it from the trap walls and directing the particles to an external chamber where the soot is burned and the ash collected for periodic disposal. As noted earlier, particulate filtration has been accomplished with high efficiency via the use of cross flow monolithic porous wall cross flow traps. When the soot and ash accumulate to unacceptable levels, the traps are regenerated by removing the soot and ash physically by a brief pulse of high pressure air (115 psi), the soot and ash being directed to a small external chamber for burning the soot and storage of the ash. While this system works effectively, the use of high pressure air increases engine fuel consumption and also requires a heavy structure, features that are undesirable for mobile applications.
The subject invention uses monolithic porous wall ceramic traps of either the cross flow or the wall flow type that will collect the soot and ash with an efficiency of 95–98%. The pressure drop across the trap increases as the soot and ash accumulate and the traps must be periodically regenerated. However, in this invention the soot and ash are removed physically by reverse flow of pre-filtered exhaust gas at normally encountered exhaust gas pressures and flow rates and the soot and ash are directed to an external chamber for burning of the soot and storage of the ash. This approach utilizes sequential periods of sustained constant reverse differential pressure that acts equally across the porous walls of all of the passages and throughout their length to dislodge and erode the collected cake of soot and ash and move the particles to the external chamber for burning or storage. Because the engine back pressure is little greater than normal, the system operates without significant increase in engine fuel consumption, the need for low sulfur fuel or increase in weight of the filter structure.
Three-way catalyst technology has been used for a number of years in S.I. automobile engines to reduce NOx emissions to a very low level. This technology uses catalysts of precious metals like platinum, rhodium, etc. to cause reaction between the CO in the exhaust and the NOx to produce N2 and CO2. However, for this process to work there must be an excess of HC(CO) in the exhaust (a lambda of 0.95–1). Because S.I. automotive engines operate at about stoichiometric air/fuel ratios, they can easily be controlled by use of an oxygen sensor to maintain a chemically correct to slightly richer mixture in the exhaust. Thus, the catalyst causes the CO and NOx to react together. This substantially eliminates the NOx from the vehicle.
However, diesel engines (and lean burn S.I. engines) operate with an excess of oxygen in the exhaust at all operating conditions. Reaction of the HC and NOx will not proceed when O2 is present. To overcome this problem, the adsorption/reduction catalyst process has been identified. The catalyst substrate is coated with a wash coat of BaO along with a catalyst of platinum, rhodium, etc., as shown with a conventional wall flow trap in FIG. 4. The trap with its catalytic coatings is operated within a temperature window of about 250–450° C.
During normal diesel lean burn operation, the NOx in the exhaust is adsorbed on the BaO coating by a process called chemisorption. This action is allowed to continue for 1 to 4 minutes. After this time, the BaO coating becomes saturated with NOx stored as Ba(NO3)2 and additional NOx will begin to pass on through the trap. However, after a time of 1 or 2 minutes, the exhaust gas is temporarily made rich (a lambda of 0.95 to 1) for a few seconds by adding additional fuel to the exhaust stream to react with the free Oxygen. This action almost instantly releases the trapped NO2 from the BaO coating and the NO2 is then immediately reacted with the CO through the action of the adjacent precious metal catalyst sites and exits to the atmosphere as N2 and CO2.
This action is capable of reducing NOx exhaust emissions by 90+%. This method is being promoted to be used in conjunction with a wall flow particulate trap. Two phenomena tend to limit the life of the device.
First, much of the sulfur in the fuel will be exhausted as SO2. This will be further oxidized in the presence of the platinum catalyst to form SO3 and this reacts with the BaO adsorber coating to form BaSO4. This poisons the trap by soon preventing the adsorption of NO2. Consequently, the adsorber-catalyst approach is predicated on the use of ultra low sulfur (10 ppm) diesel fuel. However, even this small amount of sulfur will gradually poison the trap after several thousand miles. The sulfur-poisoned trap can be regenerated by heating the trap in the presence of hydrocarbons at a temperature of about 700° C. for several minutes.
The second phenomena, with a conventional wall flow trap, is gradual plugging with the incombustible ash. After about 50,000 miles the traps must be cleaned by reverse flow, etc., as a maintenance step.
The initial part of the Detail Description/Specification will address the use of the above trap approach that uses reverse flow or reverse flow and through flow for regeneration with either cross flow and wall flow traps. These trap systems will be able to achieve the EPA 2007 particulate standards for both new and retrofit engines without need for a change in current diesel fuels.
The remainder of the Detail Description/Specification will address modifications and additions to the above particulate trap systems to use adsorber-catalyst technology that will permit achievement of the 2007 EPA particulate and the EPA NOx standards when ultra low sulfur becomes available.